What is VPD (Vapor Pressure Deficit)? Why is it Important for Your Plants?

If you want to take your breeding skills to the next level, one of the most critical concepts you need to learn is VPD (Vapor Pressure Deficit) .

Setting this value "just right" can maximize your plants' growth rate and harvest quality. However, in our experience, the VPD concept is often misunderstood, and incorrect applications unfortunately harm the plants. Let's examine what this technical term is, how to calculate it, and common mistakes in the simplest terms.

What is VPD and Why is it Important?

In its simplest terms, VPD is a measure of how "water-demanding" the air is.

Technically, it refers to the pressure difference between air saturated with 100% humidity and the surrounding air, and is usually measured in kilopascals (kPa) .

• Low VPD: Indicates that the air is saturated with water. Water evaporates very slowly.

• High VPD (Volatile Content Price): Indicates that the air is very dry. Evaporation is very rapid.

You can think of it this way for your plants:

The inside of a healthy plant's leaves is 100% moist. The VPD (Virtual Percentage Deposition) value indicates how quickly this water leaches from the leaf into the air.

• If VPD is too high: The plant transpires too quickly, cannot draw enough water from the roots, and may dry out and become stressed.

• If VPD is too low: The plant cannot transpire (transpiration stops), which halts nutrient uptake because water flow slows down. The risk of mold is high.

Ideal VPD Ranges (General Guide)

Most plants thrive in a temperature range of 0.45 to 1.25 kPa . However, these needs vary depending on the plant's life cycle. Especially in indoor cultivation (for example, medicinal plants), the following ranges can be used as a reference:

• Clones and Shoots (Rooting stage): 0.3 - 0.6 kPa (High humidity required)

• Vegetative Period (Growth Stage): 0.8 - 1.1 kPa

• Early Flowering Stage: 1.0 - 1.2 kPa

• Late Flowering Stage: 1.2 - 1.6 kPa

Important Tip: Plants don't like sudden changes. When changing VPD values (for example, when transitioning from the vegetation phase to the production phase), do so gradually over 2-3 days.

The Biggest Mistake: Room Temperature vs. Leaf Temperature

The biggest trap growers fall into when calculating VPD (Variable Percentage of Productivity) is mistaking room temperature for leaf temperature.

If you're just looking at the thermometer in the room and using the VPD chart, you're probably doing it wrong. Because leaf temperature is almost never the same as room temperature.

The factors that influence this are as follows:

1. Transpiration: Just as humans cool themselves by sweating, plants cool themselves by evaporating water from their leaves.

2. Light Source: HPS lamps heat the leaves considerably, while LEDs heat them less (but this varies depending on the spectrum).

3. Airflow: Stagnant air causes moisture to accumulate around the leaves, slowing down evaporation.

Is using an IR thermometer (laser thermometer) sufficient?

Many people simply grab an infrared (IR) thermometer, point it at a leaf, and say, "Okay, I've found the temperature." Unfortunately, this is also misleading.

Thermal imaging (FLIR) scans have shown temperature differences of up to 12°F (approximately 6-7°C) even on a single plant. The top leaves are closer to the light and therefore warm, while the lower leaves are in the shade and cool. Measuring only one point and basing your entire VPD strategy on that measurement will lead to errors.

Solution: Determining the "Offset"

Trying to measure constantly changing leaf temperatures throughout the day will be tiring and yield inconsistent results. Instead, determining a "Leaf Temperature Offset" (Difference) is the most accurate approach.

• If using LEDs: Typically, leaves are 1°C to 3°C cooler than the ambient temperature. (For example, if the room temperature is 26°C, calculate the VPD (Variable Rate of Radiation) assuming the leaf temperature is 24°C).

• If using HPS/MH: The leaves may be at or warmer than ambient temperature.

Night VPD Setting

When the lights are turned off at night, the temperature drops. To maintain VPD balance, you also need to reduce humidity. However, remember that plants close their stomata at night. Therefore, transpiration decreases, and the leaf temperature equalizes with the ambient temperature. It is safer to consider the offset value as 0 in night calculations.

For Tech Enthusiasts: VPD Calculation Formulas

If you prefer to do your own calculations instead of using ready-made tables, here's the math:

1. Finding the Saturation Vapor Pressure (SVP) in Air (Tetens Equation):

(Here, $T$ is the temperature in degrees Celsius)

2. VPD Calculation:

However, remember that to find the leaf VPD (LVPD), you should use the leaf temperature , not the air temperature, in the SVP part of the formula.

Final Words

Don't blindly follow VPD charts. If the charts recommend 70% humidity but there's a risk of mold in your room or inadequate ventilation, take precautions accordingly. VPD is a useful tool, but growing plants isn't about relying on a single piece of data; it's about understanding the environment in an integrated way.